Magnetic transducer

ABSTRACT

A miniaturized electromechanical transducer has magnets providing polarizing flux across a working gap, an electrical signal coil, an armature extending through the coil and into the working gap, and a casing of particular configuration. The casing comprises a sleeve of magnetic material, elongate in the direction of its axis, which substantially surrounds the armature, coil, and magnets, said sleeve providing a substantially uninterrupted high permeability path between the magnets.

[ 1 June 20, 1972 United States Patent Tibbetts et al.

[56] References Cited UNITED STATES PATENTS [54] MAGNETIC TRANSDUCER [72] Inventors: George C. Tibbetts; Joseph A. Sawyer, both of Camden, Maine 3,111,563 11/1963 Carlson..............................179/1l4 A Primary Evaminer-Ralph D. Blakeslee Artorney--Kenway, Jenney & Hildreth [21] Appl. No.:

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22 Claims, 8 Drawing Figures PATENTEDJum m2 SHEET 10F 3 FIG. 2

MAGNETIC TRANSDUCER This application is a continuation of Ser. No. 10,093, filed Feb. 16, 1970, now abandoned, which in turn, was a continuation of Ser. No. 638,892, filed May 16, 1967, now abandoned.

BACKGROUND OF THE INVENTION Transducers of this invention are particularly useful in hearing aids. The transducers, which are bilateral in operation, may be used as microphones to convert acoustic energy to an electrical signal or as receivers to convert electrical energy to acoustic energy.

The present trend in this segment of the electroacoustic transducer field is toward smaller units. In part, this trend has resulted from the development of components and methods for making electronic amplifiers of very small size. Thus it is desired that the transducers used with such amplifiers also be miniaturized, for example so that an entire hearing aid unit may be small enough to fit well within the pinna of the ear of the user and to extend into a substantial portion of the external auditory meatus.

As the size of the transducer is reduced, new structures must be created to obtain optimum performance in reduced size. One element of particular concern is the casing of a magnetic electromechanical transducer. Such casing must permit ease of manufacture and assembly, must be mechanically stable while having a wall thickness as small as practicable, and preferably performs one or more functions in addition to its function of protecting the inner components against mechanical destruction. For example, the casing of a magnetic transducer may contribute to the shielding of the inner components against stray magnetic flux, may provide or assist in the conduction of flux in either the signal flux field or the polarizing flux field or both, may define or assist in the formation of acoustic chambers or conduits, may assist in locating the inner components as well as any further outer components of the transducer, and may have other functions generally in the nature of a reference member with respect to which other elements function or are located.

The prior art has partially achieved such features in various ways. For example, in the parallel magnetic circuit configuration of the type disclosed in U.S. Pat. No. 2,994,016 of Raymond W. Tibbetts et al, the thin sheets 8 and 9 of magnetic material substantially bridge the space between the poles of the magnets above and below the coil and armature. The sheets are shaped to fit the coil snugly and hold it in position, while simultaneously contributing to the shielding of the inner components against stray magnetic flux as well as providing I the conduction of flux in both the signal flux field and the polarizing flux field. Such electromechanical transducer when utilized in an electroacoustic transducer is enclosed in a nonmagnetic outer housing, which outer housing also provides support for the acoustic diaphragm utilized with such electromechanical transducer. In a series magnetic circuit configuration of the type disclosed in the copending application of George C. Tibbetts, Ser. No. 168,183 filed Jan. 23, 1962, the lower case 1 and upper plate of high permeability magnetic material provide the conduction of flux in both the signal flux field and the polarizing flux field; act as a locating means for the other components; and together with the upper portion of the armature constitute a means for substantially shielding the inner components against stray magnetic flux.

SUMMARY OF THE INVENTION We have devised an improved casing for use in magnetic electromechanical transducers. Whilethe invention will be described in connection with such transducers utilized in miniaturized hearing aids, the casing of this invention is not of such limited application and may be equally useful in electromechanical transducers for other applications.

In this invention the transducer comprises a sleeve as above, further comprising at least one end closure of either magnetic or nonmagnetic material. The sleeve may be fabricated from one or more pieces of strip material blanked out and then formed into the desired configuration.

The elongate sleeve of magnetic material may replace the casing in various transducers of the prior art, including that of the Tibbetts folded armature transducers disclosed in copending application Ser. No. 168,183 of George C. Tibbetts, filed Jan. 23, 1962. The sleeve is particularly useful as a casing in combination with a three-portion armature as disclosed in copending application Ser. No. 638,878 of George C. Tibbetts et al., filed of even date. Furthermore, with such a sleeve as described above, one or more additional pairs of acoustic compliance and inertance may readily be arranged in the transducer as needed.

With a transducer utilizing the sleeve of this invention, many of the basic operations of assembly are in the direction of the axis of the sleeve, so that close mechanical tolerances and clearances, low cost of fabrication, and a very rugged and stable assembly may simultaneously be achieved. The transducer is well adapted to fabrication in very small sizes and furthermore to fabrication in elongate shapes of very small cross section.

These and further objects of the invention will become more readily apparent by reference to the accompanying specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view, with parts partially broken away to show construction of the electromechanical transducer portion of a particular embodiment of the invention;

FIG. 2 is a plan view, partially in section of an electroacoustic transducer shown as a particular embodiment of the invention, taken on the line 22 of FIG. 3;

FIG. 3 is a side elevation in section of an electroacoustic transducer taken on the line 3--3 of FIG. 2;

FIG. 4 is a plan view of another electromechanical transducer embodiment of the invention, with underlying parts indicated by dashed lines; v v

FIG. 5 is a side section of the embodiment of FIG. 4 taken on line 55 thereof;

FIG. 6 is a side sectional view of a different electromechanical transducer embodiment of the invention;

FIG. 7 is a plan sectional view of an embodiment similar to that shownin FIG. 2, with an additional acoustic chamber; and

FIG. 8 is a side elevation in section of the embodiment of FIG. 7 taken on line 8-8thereof.

GENERAL DESCRIPTION As shown in FIG. 1, a particular embodiment of an improved electromechanical transducer incorporating the invention includes a sleeve 1 of magnetic material and of substan- Accordingly we provide a transducer which comprises a j sleeve of magnetic material, elongate along an axis threading the sleeve, which substantially surrounds an armature, coil, and magnets, said sleeve providing a substantially uninterrupted high permeability path between the magnets.

tially rectangular cross section. A pair of magnets 2 and 3 are secured to opposite interior faces of the sleeve near one end thereof; each magnet is provided with a pole piece, the magnet 2 with the pole piece 4 and the magnet 3 with the pole piece 5. A working gap is formed between the pole pieces. A coil 6 is positioned in the sleeve adjacent the magnets. An armature leg 7 extends through the center of the coil and through the working gap between the pole pieces 4 and 5. The armature leg 7 isconnected at one end (the lefi end as seen in FIG. 2) to a generally U-shaped armature yoke 8. Arms 9 and 11 of the armature yoke include terminal portions 10 and 12 respective- SPECIFIC DESCRIPTION OF THE PREFERRED EMBODIMENTS More specifically, the particular embodiment of the invention shown in FIGS. 1-3 includes a sleeve 1 of high magnetic permeability material which may be formed from flat strip, and if so it has a longitudinal lap joint or seam 16 (FIGS. 1, 3). Alternatively, the sleeve may be fabricated from seamless round tubing by a forming operation or may be cut from tubing drawn to the desired shape of the sleeve cross section, or it may be fonned from two pieces of flat strip with a longitudinal seam at the top of the sleeve as well as at the bottom as shown. The sleeve shown in FIGS. I-3 is one piece and is therefore a highly effective portion of the magnetic shielding of the transducer, cooperating with other parts of the transducer to prevent external stray magnetic flux from penetrating to the inner components such as coil 6, and likewise to prevent internal flux from escaping to form an external stray magnetic field. Furthermore, by making the inner edge of the seam 16 central on either magnet 2 or 3, the seam carries negligible flux with the result that effectively none of the high permeability paths provided by the sleeve between the magnets (which include the paths in the sleeve between portions 10 and 12 and the magnets) is interrupted by the seam. Moreover, the sleeve is relatively long and throughout its extent is effective in carrying the polarizing flux provided by the magnets. Accordingly the wall of the sleeve may be made relatively thin for a specified maximum flux density in the sleeve, and a considerable saving of space results. In FIGS. 1-3 the seam is shown adjacent the magnet 3 so that the apertures carrying a flexural pivot 18 and the pin 15, and the aperture 17 if central, may be more easily blanked.

The transducer further includes a magnet assembly comprising magnets 2 and 3, with a pole piece 4 on magnet 2 and a pole piece 5 on magnet 3, and nonmagnetic shims l9 and 20 between the pole pieces, spaced from opposite edges of the armature leg 7. The pole pieces are coined so as to define tapered gaps 21 and 22 between the armature leg and facing portions of the pole pieces, as more fully disclosed in U.S. Pat. No. 3,172,022 of George C. Tibbetts. Preferably the pole pieces and shims comprise a sub-assembly that may be spot welded and brazed together, to which the magnets are secured as by adhesive; the magnet assembly may be bonded within the sleeve by adhesive.

An electrical signal coil 6, which surrounds a portion of armature leg 7, is bonded at one end to the magnet assembly; the pole pieces may be provided with notches 23 and 24 (FIGS. 2 and 3) to act as stops or catch pockets to prevent the flow of the bonding adhesive onto the coined surfaces of the pole pieces. At the other end the coil has a flange 25 which completes the support of the coil, the top and bottom of the flange being bonded by adhesive to the sleeve 1, and portions of the sides of the flange being a slip fit between arms 9 and 11 as shown in FIG. 2. As shown in FIG. 1, the remaining portions of the sides of the flange 25 are relieved to insure that adhesive will not flow by capillarity between the flange and the anus 9 and I l.

A generally U-shaped armature yoke 8 has a central aperture in its crosspiece (which includes portions 13, 14) through which passes armature leg 7 having a portion 26 formed at a right angle thereto and fastened to the armature yoke. A filler piece 27 (FIG. 3) of the same material and thickness as that of the armature leg 7 permits an adjusting pad 28, whose function will be described hereinafter, to be properly fastened to the armature assembly. The arms 9 and 11 of the armature yoke have terminal portions and 12 which are slightly widened and spaced slightly farther apart than the remaining portions of arms 9 and 11. The outer faces of the terminal portions are attached to the sleeve 1. The slight widening and slightly greater spacing provide clearance in all directions between the armature yoke and the sleeve except where attached. The various connections between elements may be made by spot welding and bran'ng as appropriate.

A flexural pivot 18 attached to the armature yoke as by spot welding, and a connecting pin 15 attached by adhesive to the otherwise free end of the armature leg, extend through apertures in sleeve 1. The end of sleeve 1 near the magnet assembly is closed by a generally U-shaped closure 29 that preferably is fabricated from high permeability magnetic material to provide magnetic shielding in conjunction with sleeve 1. Closure 29 is a slip fit in sleeve 1, is further located by abutment of its arms against the armature yoke terminal portions 10 and I2, and is attached to the sleeve by adhesive. Secured to the outer face of closure 29 by adhesive is a terminal board 30, that may conveniently be fabricated from a copper-epoxy-glass cloth laminate chemically etched to provide copper terminals 31 and 32. Internal lead wires 33 from the coil 6 extend through apertures formed in the closure 29, in the terminal board 30, and in the terminals 31 and 32, and the lead wires are secured to the copper terminals by soft solder 34. Vernier magnetic adjusting means may be incorporated, for example such as disclosed in U.S. Pat. No. 3,185,779 of Joseph A. Sawyer or US Pat. No. 3,230,426 of Joseph A. Sawyer et al.

The electromechanical transducer depicted in FIG. 1 has been described up to this point. When such electromechanical transducer is employed as the foundation for an electroacoustic transducer, further components are supplied, for example as shown in FIGS. 2 and 3, including an acoustic diaphragm and cover therefor and acoustic chambers and conduits for the passage of sound. In particular there may be a frame 35 (FIG. 3) having an upturned peripheral rim 36, a large aperture 37, and a formed boundary 38 adjacent the aperture, near flexural pivot 18. Preferably the frame 35 is located and partially attached to the, sleeve 1 by spot welding, which may be performed prior to the insertion of the inner components into the sleeve. A diaphragm preferably of the type disclosed in U.S. Pat. No. 3,166,148 of George C. Tibbetts is employed, and comprises a diaphragm portion 39, a diaphragm means surround 40, and a self-frame 41 having a peripheral rim which fits within the frame 35 and is bonded and sealed thereto at boundary 38 at one edge and to frame rim 36 at the other three edges of self-frame 41. The diaphragm portion 39 has a peripheral flat area 42, provided at opposite end points with apertures through which the flexural pivot 18 and connecting pin 15 extend slightly. Adhesive masses 43 connect the flexural pivot 18 and pin 15 to the diaphragm portion. The diaphragm portion also has a central flat area 44. Between the flats 42 and 44 the diaphragm portion 39 is formed in an arch cross-section so as to increme greatly the flexural stiffness of the diaphragm portion between the flexural pivot 18 and pin 15. In part the foregoing structure is a particular embodiment of the diaphragm and translating member means disclosed in the copending application of Joseph A. Sawyer et al., Ser. No. 638,926, filed of even date, which may be consulted for a full description of the advantages and mode of operation of a pivoted diaphragm connected to the extremity of an armature or other translating means which operates in flexure or torsion.

The diaphragm is protected and a confined space above the diaphragm is provided by a cover 45 located by the outside of rim 36 of frame 35. Adhesive may be applied about the junction of the rim 36 with sleeve 1, the cover 45 being placed in position thereafter, and the adhesive bonding the cover to the frame and to the sleeve and serving to seal the frame and cover to the sleeve. The frame 35 and cover 45 may be fabricated from magnetic material to provide somewhat increased magnetic shielding against stray flux traversing the apertures containing the flexural pivot 18 and connecting pin 15.

The cover may be provided with an aperture in its major surface, for passage of sound therethrough, for example if the electroacoustic transducer is to be utilized as a microphone with a top inlet. In a different form the cover may be solid, as shown in FIG. 3, with the sound passing through various chambers and conduits and thence through the end wall of the transducer. In either form the sleeve 1 has the advantage of enabling the provision of useful acoustic chambers and passageways. Thus one end of the sleeve may be provided with a generally U-shaped bulkhead 46 (FIG. 2) acoustically closing the sleeve near that end, and the bulkhead may have a pierced hole 47 that carries an adhesive mass 48 having a fine aperture 49 therethrough. The aperture 49 serves to equalize the static pressure on each side of the diaphragm, and altematively could be formed directly through the wall of the bulkhead; the use of an adhesive mass merely provides a more convenient method of forming an aperture of small and controlled size. Before the bulkhead 46 is inserted, the position of the free end of armature leg 7 may be adjusted in the working gap between pole pieces 4 and 5 by way of adjusting pad 28, wherein the adjusting pad is engaged by a suitable tool that exerts sufficient torque on the pad to inelastically twist crosspiece portions 13 and 14. After the bulkhead 46 is located within the sleeve 1 the arms of the bulkhead may be spot welded to the sleeve. Since the crosspiece of the armature yoke substantially closes the sleeve near the bulkhead and is of high magnetic permeability material, the crosspiece acts to intercept magnetic flux whether external or internal to the transducer. Thus bulkhead 46 may be of non-magnetic material such as appropriate nickel-chromium or nickel-copper alloys.

An acoustic closure 50 that is also generally U-shaped (FIG. 3) has a short tubular drawn portion 51 that may carry an acoustic connector 52 and a sleeve 53 having an aperture 54 which defines an acoustic inertance and which may in addition contain a porous acoustic impedance means. The closure 50 is narrowed beyond its face so that its arms may fit widthwise between the arms of the bulkhead 46, (FIG. 2), its arms fitting against the interior of sleeve 1 (FIG. 3). The upper arm of closure 50 is provided with a notch 55 that leaves the aperture 17 in the sleeve unobstructed when the closure is fitted in place. The assembly is completed by applying sufficient adhesive to the accessible junctions of the bulkhead 46 with sleeve 1, and thereafter slipping closure 50 into the sleeve until the inward face of the closure abuts the ends of the arms of the bulkhead. The adhesive spreads by capillary action throughout the joints between the bulkhead, sleeve, and closure, and when cured seals the bulkhead and closure to the sleeve and bonds the closure in place. The closure 50 may also be of nonmagnetic material.

An aperture 56 is provided through the frame 35 in line with aperture 17 to permit passage of sound between the space 57, defined by the cover and diaphragm, and the space 58 defined by the bulkhead and the closure. Space 58 provides an acoustic compliance, and an acoustic inertance is provided by the aperture comprising the apertures 17, 55, 56. Such space may be further divided, as disclosed hereinafter concerning FIGS. 7 and 8. Rather than an end sound passage 51-54 in the closure as shown, acoustic connection may be made to any of the available sides of the space 58, or alternatively may be made to the space 57 above the diaphragm, for example by means of an aperture as discussed above. In the latter case space 58 may function solely as an acoustic compliance without acting as a sound passageway. Likewise, the bulkhead 46 and the closure 50 may be replaced by a single acoustic closure, or the aperture 17 may be modified to be similar to aperture 49 for venting purposes or may be replaced by a conventional Thuras tube.

It is evident from the preceding description that the sleeve of this invention permits ease of manufacture and assembly of the several components into a complete electromechanical transducer, actsas locating means for most of the components, and provides substantial magnetic shielding means for the inner components such as the coil, in addition to providing the normal casing function of protecting the inner components against mechanical destruction. The steps of assembly described above are elaborated in somewhat greater detail in the copending application Ser. No. 638,878 of George C. Tibbetts et al., referred to previously.

The operation of the electromechanical transducer of FIG. 1, and the electroacoustic transducer of FIGS. 2 and incorporating such electromechanical transducer, will be obvious to the artisan. When the device of FIG. 1 for example is operated to transduce mechanical to electrical energy, as in a microphone or phonograph pick-up, mechanical vibration impressed upon pin 15 causes the armature leg 7 to move between pole pieces 4 and 5, unbalancing the magnetic circuit and causing flux (provided by the magnets) to flow alternately along the armature in opposite directions along the closed paths formed by the armature leg 7 and armature yoke 8, sleeve 1, and either magnet 2 or 3 with respective pole pieces 4 or 5 and air gaps 21 or 22. The alternating flux in armature leg 7 induces an alternating e.m.f. in coil 6, and the e.m.f. may be led via terminals 31 and 32 to amplifying or other means as desired.

When operated to transduce electrical energy to mechanical energy, an electrical signal current through terminals 31 and 32 is conducted by the coil 6, which causes magnetic signal flux to flow in armature yoke 8v and armature leg 7, resulting in a net magnetic tractive force on armature leg 7 toward either pole piece 4 or 5 because of the addition (or subtraction) of a portion of the signal flux to the polarizing flux in gap 21 and the subtraction (or addition) of the other portion of the signal flux to the polarizing fiux in gap 22. The resulting mechanical deflection of the armature assembly is transmitted by pin 15 to further components, for example a diaphragm as shown in FIGS. 2 and 3.

The sleeve of the present invention also is useful as a casing for a folded armature magnetic transducer as disclosed in the copending application of George C. Tibbetts, Ser. No. l68,183 filed .Ian. 23, 1962 referred to previously, wherein the folded armature preferably plays an essential role in the shielding of the inner components of the transducer, particularly the coil, the free end of the armature, and the magnets.

The particular embodiment shown in FIGS. 4 and 5 comprises a sleeve 60 of high permeability magnetic material,

elongate in the direction of its axis. The sleeve is formed from one piece of flat strip and has a lap joint 61 extending lengthwise of the sleeve and centered in the bottom. The sleeve has a substantial perforation 62, i.e., an opening bounded on all sides, through the wall of the sleeve. Folded armature 63 has a portion 64 attached to the sleeve at 65, the portion extending along the sleeve in the opening 62 and substantially closing the opening against passage of stray magnetic flux therethrough. Closure portion 64 is integral with a connecting portion 66 which in turn is integral with a portion 67 extending through coil 68 into the working gap, including tapered gaps 69, 70, of magnet assembly 71. The magnet assembly comprises magnets 72 and 73, pole pieces 74 and 75, and nonmagnetic shims between the pole pieces, spaced from opposite edges of the armature portion 67. The pole pieces 74 and 75 are coined so as to define the tapered gaps 69, 70

between the armature portion 67 and facing portions of the pole pieces, as more fully disclosed in US. Pat. No. 3,172,022 of George C. Tibbetts. A flange 76, or other part of the electrical signal coil 68, fits within the sleeve 60 and is bonded thereto by adhesive. The magnet assembly is preassembled outside of the sleeve (as described concerning FIGS. 1-3) and fits within the sleeve in the direction shown as vertical in FIG. 5, being bonded thereto by adhesive. The coil may also be bonded by adhesive to the magnet assembly if such means is desired to provide support for that end of the coil.

The end of sleeve 60 near the magnet assembly 71 may be closed by a generally U-shaped closure 77 and a terminal board 78. Preferably the closure 77 is fabricated of high permeability magnetic material to act as magnetic shielding means in conjunction with sleeve 60. Closure 77 is a slip fit in sleeve 60 and is bonded within the sleeve by adhesive. As described with reference to FIGS. 1-3, terminal board 78 may be fabricated from a copper-epoxy-glass cloth laminate,

chemically etched to provide copper terminals 79 and 80. In-

ternal lead wires 81 from the coil 68 extend through apertures in the closure 77, terminal board 78, and terminals 79 and 80 and are connected and secured to the terminals by soft solder 82. Vernier magnetic adjusting means may also be incorporated in this embodiment, as disclosed above.

The other end of sleeve 60, near the coil 68, may be closed by another generally U-shaped closure 83, fitting within the sleeve and bonded thereto by adhesive. The closure 83 preferably is of high permeability magnetic material, since in this embodiment the armature portion 66 does not substantially close a cross section of the sleeve. A flexural pivot 84 for a pivoted diaphragm is attached to the inside of the sleeve and extends up through the end of aperture 62 adjacent portion 66 of the armature, and a connecting pin 85 is fitted up through another aperture 86 in the sleeve, the base of the pin 85 being positioned on the otherwise free end of armature portion 67 and bonded thereto by adhesive. As stated above, copending application Ser. No. 638,926 of Joseph A. Sawyer et al., filed of even date may be consulted for a full description of the advantages and mode of operation of a pivoted diaphragm con nected to the extremity of an armature or the like. Such diaphragm, not shown in FIGS. 4 and 5, as well as a cover for the diaphragm and any associated acoustic conduits, may be provided on the top side of the sleeve as appropriate or as indicated for the previous embodiment shown in FIGS. 2 and 3.

The operation of the balanced armature transducer embodiment of FIGS. 4 and 5 will be obvious to the artisan as described above for FIGS. 1-3. Because the armature is free of attachment to the sleeve except at 65 adjacent the magnet assembly, the state of adjustment of the armature portion 67 in the working gap of the magnet assembly 71 is not greatly af fected by incidental rotation of the armature 63 relative to the sleeve 60 about an axis (normal to the plane of FIG. 5) through the region of the joint at 65, as more fully discussed in copending application Ser. No. 168,183 of George C. Tibbetts and copending application Ser. No. 638,878 of George C. Tibbetts et al referred to previously. This is also true of the embodiments of FIGS. 1-3, 6, and 7-8 having the armature free of attachment to the sleeve except at regions adjacent the magnets and the working gap therebetween.

In the prior folded armature utilization, disclosed in copending application Ser. No. 168,183 noted above, the fixed extremity of the armature was attached as by welding and brazing to an apertured plate, which in turn was attached by welding within a cup shaped casing. In such structure the signal flux follows two types of paths corresponding to considerably different parasitic reluctances. That is, one type of path extended between the fixed end of the armature and the upper magnet through two joints, namely the joint to the plate and the joint between the plate and the upper magnet; whereas the other type extended between the fixed end of the armature and the lower magnet through three joints, namely the joint to the plate, the joint between the plate and the cup, and the joint between the cup and the lower magnet. Furthermore, the polarizing flux completed its path through the joint between the plate and the cup. In the present invention utilizing sleeve 60, all signal flux paths are similar in that each extends through only two joints to either magnet, namely through the joint at 65 and then through a joint between the sleeve 60 and either magnet. As noted above with reference to FIGS. 1-3, a seam in the sleeve carries negligible flux if the seam is made longitudinally and central on either magnet. Hence such seam or seams effectively does not interrupt any high permeability path for either signal or polarizing flux.

In the embodiment of FIG. 6 the armature 96 is again folded as in FIGS. 4-5, but is entirely within the sleeve (as in FIGS. 1-3) with no opening in the wall of the sleeve near the at tachment of the annature to the sleeve. Elongate sleeve 87 of high permeability magnetic material has a lap joint 88. Within the sleeve is a magnet assembly 89 similar to those previously described, comprising upper and lower magnets 90 and 91, pole pieces 92 and 93 defining tapered gaps 94 and 95 between the upper and lower surfaces of the armature portion therebetween and the slanting face of each respective pole piece, and nonmagnetic shims between the pole pieces spaced from opposite edges of the armature portion. Folded armature 96 has two spaced but otherwise coextensive portions 97, 98 folded upon each other and joined by an intermediate connecting portion 99 integral with 97 and 98. The armature portion 98 extends in the working gap between the pole pieces. The annature portion 97 extends along but spaced from the sleeve, to provide room for its vibratory motion, and its extremity 100 is offset outwardly to form a joint with the sleeve in the area between the sleeve and the bottom of the magnet assembly. Surrounding the armature portion 98 is an electrical signal coil 101 attached to the upper inside wall of sleeve 87. Since the armature portion 97 is within the sleeve, rather than in an opening in the wall thereof as in FIGS. 4 and 5, the coil 101 and armature portion 98 are preferably off center in the sleeve to allow room for motion of armature portion 97 without the wasteful provision of another space or spacer on the other side of the coil 101, and thus the magnet assembly 89 is placed correspondingly off center. Thus between magnet 91 and extremity 100 is a shim 102 of magnetic material that provides support for the magnet assembly and completes the flux path between the shell 87 and lower magnet 91. A separate shim such as 102 is desirable in that all signal flux paths contain exactly two joints between the extremity 100 and either magnet.

The remaining components of the electromechanical transducer of FIG. 6 are similar to those comparable elements described above for the embodiments of FIGS. 1-5. The end of the sleeve near portion 99 may be closed by a cup-shaped closure 103, preferably of high permeability magnetic material, having a rim 104 about its periphery, the rim being a slip fit within the sleeve 87 and attached thereto as by adhesive, the lower portion of the rim 104 being partially relieved as at 105 to assure clearance for armature portion 97. At the other end of the sleeve 87 is another cup-shaped closure 106 having a rim 107 about its periphery, the rim similarly fitting in the sleeve and attached thereto by adhesive. Attached to the outer face of closure 106 is a terminal board 108 of copper-epoxyglass cloth laminate, the copper having been etched off everywhere except at two terminals such as the one 109 shown. Lead wires 1 10 extend from the coil 101 through apertures in closure 106, terminal board 108, and terminals 109, and are secured to the terminals by soft solder 111. A connecting pin 112 is attached to armature portion 98 and extends in a space between coil 101 and magnet assembly 89 up through an aperture 1 13 in sleeve 87 The connecting pin may be of one, piece, or for ease of assembly may comprise two pieces as shown: a strap 114 which girdles the armature portion 98 and has two ends extending side by side up from portion 98, and a tube 115 which is fitted over the ends of strap 114 at a later stage of assembly, and bonded thereto by adhesive, the same adhesive serving to bond strap 114 to portion 98. The ends of the strap 114 may be spot welded together before the strap is pushed onto armature portion 98 from its free end.

As noted above concerning FIGS. 2 and .3, an acoustic com pliance chamber near an end of the sleeve, with or without the function of a sound passageway, may be divided into two or more such spaces. In other words, the sleeve of this invention is adaptable to provide one or several acoustic inertances and compliances within itself. A transducer according to any of the foregoing embodiments may be so modified, and FIGS. 7 and 8 show for convenience a particular modification at one end of the form of transducer depicted in FIGS. 1-3. In FIGS. 7 and 8 elongate sleeve 117 (which may be seamless as shown) substantially surrounds an armature 118 having a generally U-shaped armature yoke 119 with arms 120 and 121 and crosspiece 122, an armature leg 123 extending through and from the crosspiece perpendicular thereto, an end 124 of the armature leg bent at a right angle and attached to the crosspiece, a filler piece 125 attached to the crosspiece, and an adjusting pad 126 attached to end 124 and filler piece 125. The sleeve, armature yoke arms, and armature leg are shown broken away at the right of the figures, to emphasize that the modification is at the left end of the transducer.

A generally U-shaped bulkhead 127 has arms (FIG. 7) that fit within and may be spot welded to the sleeve, the bulkhead having a pierced hole 128 that carries an adhesive mass 129 having a fine aperture 130 therethrough for venting purposes. A generally U-shaped dividing wall 131 has upper and lower arms (FIG. 8) that fit within the sleeve and the arms of the bulkhead, and abut the bulkhead, the wall being attached and sealed to the sleeve and bulkhead by adhesive. Wall 131 has an aperture 132 therethrough, and the upper arm of dividing wall 131 is notched at 133 so that the arm does not obstruct the sound conduit 134 through the sleeve wall. A generally U- shaped outer closure 135 has upper and lower arms that fit within the sleeve and abut the face of dividing wall 131, the closure being secured and sealed to the sleeve and wall by adhesive. Closure 135 has a tubular drawn portion 136 having an aperture 137 therein for conduction of sound. Further acoustic impedance means may be provided in aperture 137. Bulkhead 127 and dividing wall 131 define a space 138 therebetween, and dividing wall 131 and closure 135 define a space 139 therebetween, which spaces function as acoustic compliances as well as sound passageways, while apertures 134, 132, and 137 provide acoustic inertances and associated acoustic resistances. Further dividing walls, which may be apertured, may be employed for the purpose of providing more than two acoustic compliances within an end space of the sleeve. Although the spaces may be interconnected differently; and the placement of the acoustic outlet or inlet may differ, the type of acoustic network described herein is particularly useful in providing both design flexibility and improved performance in the electroacoustic transducer.

Many modifications, choice of materials and the like will be obvious to the artisan. However the invention is to be limited only by the scope of the following claims.

We claim: 1. A magnetic transducer comprising a pair of magnets having a working gap therebetween, an electrical signal coil, an armature extending in a first direction through said coil into said gap and having a vibratable portion in said gap,

and a sleeve comprising one piece of high magnetic permeability material fabricated to encircle an axis and to be elongate along the axis, said axis extending in said first direction, said sleeve interconnecting said pair of magnets and substantially surrounding said coil, said armature and said magnets.

2. A magnetic transducer as in claim 1 wherein said armature has a second portion extending to a joint with said sleeve, said joint having low reluctance to magnetic signal flux.

3. A magnetic transducer as in claim 1 further comprising a closure at an end of said sleeve.

4. A magnetic transducer as in claim 3 wherein said closure comprises high magnetic permeability material.

5. A magnetic transducer as in claim 3 further comprising a bulkhead within said sleeve and substantially partitioning the sleeve.

6. A magnetic transducer as in claim 5 further comprising an element at the end of said sleeve spaced from said bulkhead to define an acoustic compliance chamber.

7. A magnetic transducer as in claim 6 wherein said acoustic compliance chamber has an aperture in one of its walls to the exterior of the transducer, for conduction of sound therethrough.

8. A magnetic transducer as in claim 6 wherein said element is apertured for conduction of sound.

9. A magnetic transducer as in claim 6 further comprising at least one additional wall dividing said acoustic compliance chamber into at least two acoustic compliance chambers.

10. A magnetic transducer as in claim 9 further comprising acoustical impedance means interconnecting at least one pair of acoustic compliance chambers.

11. A magnetic transducer as in claim 6 further comprising an acoustic diaphragm connected to said vibratable portion of the armature, said diaphragm being outside of and spaced from said sleeve, and a cover over and spaced from said diaphragm and attached to said sleeve, the space between the cover and diaphragm being connected by acoustic impedance 2 means through the wall of the sleeve to said acoustic compliance chamber.

12. A magnetic transducer as in claim 2 wherein said second portion is vibratable, and said armature is folded in that said vibratable portion and said second portion face and are spaced from each other.

13. A magnetic transducer as in claim 12 wherein said second portion has an extremity extending into a gap between one magnet and said sleeve, and a shim of magnetic material lies between said magnet and said extremity.

14. A magnetic transducer comprising a pair of magnets having a working gap therebetween,

an electrical signal coil,

an armature extending through said coil and having a vibratable portion in said gap,

and a sleeve elongate along an axis threading the sleeve and being fabricated in one piece of high magnetic permeability material, said sleeve interconnecting said pair of magnets, said sleeve having a substantial perforation through the wall thereof, said armature comprising a portion attached to the sleeve and extending along said perforation to substantially close the perforation, said sleeve and said second-named portion of the armature constituting mean substantially surrounding said coil, said magnets, and said first-named portion of the armature.

15. A magnetic transducer as in claim 14, wherein said second-named portion is vibratable, and said armature is folded in that said first-named portion and said second-named portion face and are spaced from each other.

16. A magnetic transducer comprising a pair of magnets having a working gap therebetween, each magnet having a pole surface facing away from the working gap, said pole surfaces being of opposite polarity,

an electrical signal coil,

an armature extending through said coil and having a vibratable portion in said gap,

and a longitudinally seamless sleeve, elongate along an axis threading the sleeve, having surface portions facing toward each of said pole surfaces respectively, the sleeve substantially surrounding said coil, armature and magnets, said sleeve comprising a direct unbroken high magnetic permeability path between said surface portions substantially throughout the extent of the sleeve.

17. A magnetic transducer as in claim 16 wherein said armature has a second portion extending to a joint with said sleeve, said joint having low reluctance to magnetic signal flux.

18. A magnetic transducer having, in combination,

a tubular longitudinally seamless sleeve of high magnetic permeability material having a closed annular cross section substantially throughout the length of the sleeve and having open ends,

a pair of magnets having a working gap therebetween, each magnet being within the sleeve and connected thereto by means having low reluctance to magnetic flux,

an electrical signal coil within the sleeve,

and an armature within the sleeve, the armature extending through said coil and having a vibratable portion in said 19. A transducer according toclaim 18, in which the signal coil is supported by attachment to the inner surface of the sleeve.

20. A transducer according to claim 18, in which the armature is attached to the sleeve by a joint having low reluctance to magnetic signal flux.

21. A transducer according to claim 20, in which the armature has a second portion which partitions for magnetic shielding purposes the interior of said sleeve.

22. A magnetic transducer comprising a pair of magnets having a working gap therebetween, an electrical signal coil,

ture and said magnets, said armature extending to a joint of low magnetic reluctance with said sleeve, and having a second portion which for magnetic shielding purposes substantially closes said opening.

UNlTED STATEEB PATENT @FFIUE 569 crmucirr or coREcmN Patent No. ,671,684 Dated June 20, 1972 Inventor) Georg C. Tibbetts and Joseph A. Sawyer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 75, after "and" (second occurrence) insert -3--. Column 10, line 27, cancel "mean" and substitute --means--.

Signed and sealed this 3rd day of October 1972.

(SEAL) Attest:

ROBERT GO'ITSCHALK EDWARD MQFLETCI-IER,JR. Attesting Officer Commissioner of Patent 

1. A magnetic transducer comprising a pair of magnets having a working gap therebetween, an electrical signal coil, an armature extending in a first direction through said coil into said gap and having a vibratable portion in said gap, and a sleeve comprising one piece of high magnetic permeability material fabricated to encircle an axis and to be elongate along the axis, said axis extending in said first direction, said sleeve interconnecting said pair of magnets and substantially surrounding said coil, said armature and said magnets.
 2. A magnetic transducer as in claim 1 wherein said armature has a second portion extending to a joint with said sleeve, said joint having low reluctance to magnetic signal flux.
 3. A magnetic transducer as in claim 1 further comprising a closure at an end of said sleeve.
 4. A magnetic transducer as in claim 3 wherein said closure comprises high magnetic permeability material.
 5. A magnetic transducer as in claim 3 further comprising a bulkhead within said sleeve and substantially partitioning the sleeve.
 6. A magnetic transducer as in claim 5 further comprising an element at the end of said sleeve spaced from said bulkhead to define an acoustic compliance chamber.
 7. A magnetic transducer as in claim 6 wherein said acoustic compliance chamber has an aperture in one of its walls to the exterior of the transducer, for conduction of sound therethrough.
 8. A magnetic transducer as in claim 6 wherein said element is apertured for conduction of sound.
 9. A magnetic transducer as in claim 6 further comprising at least one additional wall dividing said acoustic compliance chamber into at least two acoustic compliance chambers.
 10. A magnetic transducer as in claim 9 further comprising acoustical impedance means interconnecting at least one pair of acoustic compliance chambers.
 11. A magnetic transducer as in claim 6 further comprising an acoustic diaphragm connected to said vibratable portion of the armature, said diaphragm being outside of and spaced from said sleeve, and a cover over and spaced from said diaphragm and attached to said sleeve, the space between the cover and diaphragm being connected by acoustic impedance means through the wall of the sleeve to said acoustic compliance chamber.
 12. A magnetic transducer as in claim 2 wherein said second portion is vibratable, and said armature is folded in that said vibratable portion and said second portion face and are spaced from each other.
 13. A magnetic transducer as in claim 12 wherein said second portion has an extremity extending into a gap between one magnet and said sleeve, and a shim of magnetic material lies between said magnet and said extremity.
 14. A magnetic transducer comprising a pair of magnets having a working gap therebetween, an electrical signal coil, an armature extending through said coil and having a vibratable portion in said gap, and a sleeve elongate along an axis threading the sleeve anD being fabricated in one piece of high magnetic permeability material, said sleeve interconnecting said pair of magnets, said sleeve having a substantial perforation through the wall thereof, said armature comprising a portion attached to the sleeve and extending along said perforation to substantially close the perforation, said sleeve and said second-named portion of the armature constituting mean substantially surrounding said coil, said magnets, and said first-named portion of the armature.
 15. A magnetic transducer as in claim 14, wherein said second-named portion is vibratable, and said armature is folded in that said first-named portion and said second-named portion face and are spaced from each other.
 16. A magnetic transducer comprising a pair of magnets having a working gap therebetween, each magnet having a pole surface facing away from the working gap, said pole surfaces being of opposite polarity, an electrical signal coil, an armature extending through said coil and having a vibratable portion in said gap, and a longitudinally seamless sleeve, elongate along an axis threading the sleeve, having surface portions facing toward each of said pole surfaces respectively, the sleeve substantially surrounding said coil, armature and magnets, said sleeve comprising a direct unbroken high magnetic permeability path between said surface portions substantially throughout the extent of the sleeve.
 17. A magnetic transducer as in claim 16 wherein said armature has a second portion extending to a joint with said sleeve, said joint having low reluctance to magnetic signal flux.
 18. A magnetic transducer having, in combination, a tubular longitudinally seamless sleeve of high magnetic permeability material having a closed annular cross section substantially throughout the length of the sleeve and having open ends, a pair of magnets having a working gap therebetween, each magnet being within the sleeve and connected thereto by means having low reluctance to magnetic flux, an electrical signal coil within the sleeve, and an armature within the sleeve, the armature extending through said coil and having a vibratable portion in said gap.
 19. A transducer according to claim 18, in which the signal coil is supported by attachment to the inner surface of the sleeve.
 20. A transducer according to claim 18, in which the armature is attached to the sleeve by a joint having low reluctance to magnetic signal flux.
 21. A transducer according to claim 20, in which the armature has a second portion which partitions for magnetic shielding purposes the interior of said sleeve.
 22. A magnetic transducer comprising a pair of magnets having a working gap therebetween, an electrical signal coil, an armature extending through said coil and having a vibratable portion in said gap, and a sleeve comprising one piece of high magnetic permeability material fabricated to encircle an axis and to have an opening extending from one extremity to the other along said axis, said sleeve interconnecting said pair of magnets and substantially surrounding said coil, said armature and said magnets, said armature extending to a joint of low magnetic reluctance with said sleeve, and having a second portion which for magnetic shielding purposes substantially closes said opening. 